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In a word, ratings. Wikipedia has a nice table of per-episode ratings. After the premiere, the ratings plunged (Note that the first 4 episodes aired on Sundays. After that, one episode aired on Saturday. Then the rest was shelved until the summer). After moving Kings off Sunday night, NBC saw higher ratings in that timeslot.

In addition, it was not cheap to make, at $4 million an episode. Personally, I think the show's poor marketing didn't help things. Watching ads on TV leading up to the premiere didn't make it clear what Kings was or what it was about. I only knew because I read up online about the show, and found the premise of 'bible story in the modern day' to be interesting.